Geometrically optimized fast neutron detector

ABSTRACT

An improved fast neutron detector fabricated with alternating layers of hydrogenous, optically transparent, non scintillating material and scintillating material. Fast neutrons interact with the hydrogenous material generating recoil protons. The recoil protons enter the scintillating material resulting in scintillations. The detector is optically coupled to a photomultiplier tube which generates electrical pulses proportional in amplitude to the intensity of the scintillations, and therefore are an indication of the energy of the fast neutrons impinging upon the detector. Alternating layers of materials are dimensioned to optimize total efficiency of the detector, or to optimize the spectroscopy efficiency of the detector. The scintillating material is preferably ZnS, and the hydrogenous material is preferably plastic. The detector is ideally suited for well logging applications and fast neutron monitor applications.

RELATED APPLICATIONS

[0001] This application is related to application Ser. No.______entitled “Geometrically Optimized Fast Neutron Detector” andapplication Ser. No. ______entitled “Geometrically Optimized FastNeutron Detector”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the invention

[0003] This invention is directed toward an improved fast neutrondetector, and more particularly directed toward the optimization of thedetector efficiency when used in logging of earth formations penetratedby a borehole and for a variety of applications.

[0004] 2. Background of the Art

[0005] In the context of this disclosure, “logging” is defined as themeasure of a parameter of material penetrated by a borehole, as afunction of depth within the borehole.

[0006] There are many types or classes of borehole logging systems.These classes include, but are not limited to, electromagnetic, acousticand nuclear systems. Each class of logging system typically comprises a“source” which emits energy into the surrounding formation, and one ormore “detectors” which measure energy returning from the formation.Detector responses, when properly analyzed and processed, yieldformation and borehole parameters of interest.

[0007] Any type or class of logging system typically comprises a sourceand detector system with sufficient depth of investigation to penetratethe logging instrument housing, penetrate the immediate borehole region,enter the surrounding earth formation, interact with the formation, andinduce some type of response which returns to the borehole and thelogging instrument to be detected and analyzed. Nuclear logging systemstypically involve the use and measure of gamma radiation and neutronradiation. These types of beta particles. As a result, nuclear logginginstruments typically comprises a source of neutrons, or a source ofgamma radiation, one or more neutron detectors, or one or more gamma raydetectors, or some combination of these different types of sources anddetectors.

[0008] Logging instruments are typically conveyed along a borehole bymeans of a wireline of drill string thereby creating a “log” offormation parameters as a function of depth within the borehole.Borehole conditions are harsh in that temperatures and pressures arehigh. Components within a logging instrument, such as detectors, aresubjected to these environmental conditions well as vibration andimpacts resulting from the conveying of the instrument along theborehole. As an example, nuclear detectors used in logging applicationmust be able to withstand these harsh conditions of the boreholeenvironment including temperatures which can reach 175 degreesCentigrade (°C.) or higher.

[0009] All nuclear logging systems involve the measure of statisticalnuclear processes. As a result, statistical significance of themeasurements is of prime importance since it directly affects thestatistical precision of one or more parameters of interest computedfrom the measurement. Statistical precision improves as the number ofdetector events increases. It is therefore very desirable to maximizethe efficiency of nuclear detectors used in borehole logging operations.Further, space is often limited in downhole instrumentation making it ofutmost importance to maximize detector efficiency for a given geometryallowed in the design of the instrument.

[0010] Attention will now be directed toward prior art neutrondetectors. Liquid scintillators have been used to detect high energy or“fast” neutrons. These scintillators also respond to impinging gammaradiation. Neutron and gamma ray “events” generate different pulse shaperesponses from liquid scintillators. Pulse shape discrimination methodstherefore provide means for separating fast neutron and gamma rayinduced responses in liquid scintillator detectors. Fast neutron andgamma ray measurements can be made with a single liquid scintillatordetector. Liquid scintillators are relatively efficient. Unfortunately,liquid scintillators consist of flammable mixtures, and some mixtureshave very low flash points. For these reasons, liquid scintillators arenot desirable for high temperature, high pressure downhole applications.

[0011] Gas filled detectors, such as detectors containing relativelyhigh pressures of helium-4 (⁴He), have been used as fast neutrondetectors. These detectors are relatively rugged, and can withstandrelatively high temperatures encountered within the borehole. Becausethe detectors are gas filled rather than liquid or solid, theirdetection efficiency is relatively low, and therefore not particularlydesirable for downhole applications where statistical significance ofmeasured detector response is of prime importance.

[0012] Plastic scintillators are relatively efficient neutron detectors,rugged in construction, and able to operate at temperatures of at least175° C. These detectors are, however, responsive to both fast neutronsand gamma radiation. Neutron and gamma ray events can not be delineatedby the shape or amplitude of pulses produced by the detector. Thecrystal anthracene, a hydrocarbon, is another type of solid materialused in fast neutron detectors but, like the plastics, can not separatefast neutron from gamma ray events using pulse shape or pulse amplitudediscrimination.

[0013] Stilbene and p-terphenyl crystals are fast neutron detectors andare reported to produce pulses which can be separated into fast neutronand gamma ray events. This class of detector does not have theflammability of the liquid scintillators. The crystals are, however, notrated as operable at temperatures of 175° C. The crystals are alsodifficult to fabricate, and availability is questionable with the onlyknown source being Russia.

[0014] A fast neutron detector potentially suitable for downholeapplications is an activated zinc sulfide scintillator combined with anonscintillating plastic. The activated dopant is preferably silver (Ag)but other elements, such as copper (Cu) may be suitable or even betteractivators depending on the application of the detector. Activated zincsulfide will be denoted by the symbol “ZnS” in the remainder of thisdisclosure, with the understanding that the dopant can consist of avariety of materials. The non scintillating plastic can be any hydrogenrich material that is optically transparent and that possesses suitablemechanical properties.

[0015] Geometrically, the detector is constructed with a ZnS cylindricalcore surrounded by alternating and concentric cylinders of plastic andZnS. The scintillator detector was first introduced by Emmerich in 1954(W. S. Emmerich., Review of Scientific Instruments, vol. 25, page 69(1954)). Neutron and gamma ray events can be separated by pulseamplitude discrimination. Fast neutron detectors of this type areoffered commercially by the Bicron division of Saint-GobainInternational Ceramics, Inc. The material in not flammable, and it isthought that the detector can meet a 175° C. temperature rating withsome modifications. The main disadvantage of this type of detector forborehole applications is the relatively small volume, with correspondingreduction in detector efficiency. Furthermore, efficiency is notmaximized for specified detector volumes, and in particular forspecified detector geometry of diameter D and length L. Detector volumeis restricted by the lack of light transparency of ZnS, withscintillations within the ZnS element only being able to reach anoptically coupled photomultiplier (PM) tube through the transparentplastic component of the detector. The plastic component of the detectorcontains hydrogen (H). As with other H containing fast neutrondetectors, the material responds to fast neutrons impinging upon thedetector by the proton recoil process, with recoil protons generatingscintillations within the ZnS component of the detector. Detectorresponse is further enhanced by a threshold (n,p) reaction with ³²S asreported by Birks (J. B. Birks, The Theory and Practice of ScintillationCounting, Pergamon Press, page 548, Oxford, 1964). This reactionintroduces additional neutron induced proton flux within the ZnSscintillation material thereby increasing the efficiency of thedetector.

[0016] Measures of fast neutrons are used in many prior art well loggingsystems to determine formation and borehole parameters of interest. Inthese prior art systems, fast neutron fluxes are typically measuredinefficiently, and in many cases are determined indirectly in that theother parameters are measured and used to compute fast neutron fluxes.

[0017] The prior art contains patents teaching various apparatus andmethod for measuring and applying neutron and gamma ray measurements toobtain parameters of earth formations penetrated by a borehole. Patentsthought to be the most relevant to this disclosure are summarized asfollows:

[0018] U.S. Pat. No. 4,122,339 to Harry D. Smith, Jr. et al discloses alogging system that irradiates, with fast neutrons, earth formationspenetrated by a borehole. Fast neutron population is measured indirectlyfrom inelastic scatter gamma radiation detected with a gamma raydetector during bursts of fast neutrons from a pulsed neutron source. Anepithermal neutron detector is used to measure epithermal neutronpopulation following each neutron burst. The inelastic scatter gamma raymeasurement is then combined with a fast neutron/epithermal neutronratio to determine formation porosity.

[0019] U.S. Pat. No. 4,122,340 to Harry D. Smith, Jr. et al discloses alogging system using epithermal and fast neutron detectors. A stilbenescintillation crystal is used to detect fast neutrons. Measurements offast and epithermal neutrons are combined to determine formationporosity.

[0020] U.S. Pat. No. 4,134,011 to Harry D. Smith, Jr. et al discloses alogging system comprising one epithermal and one fast neutron detector.Formation porosity is determined by making a dual spaced fast toepithermal neutron measurement using a continuous source of fastneutrons. Stilbene is used in the fast neutron detector with a spacingfrom the neutron source of 40 to 80 centimeters (cm). Pulse shapediscrimination is used to separate gamma ray events from fast neutronevents.

[0021] U.S. Pat. No. 4,152,590 to Harry D. Smith, Jr. et al discloses alogging system which is very similar to the system disclosed in U.S.Pat. No. 4,134,011 summarized above. A thermal decay rate measurement isadded.

[0022] U.S. Pat. No. 4,605,854 to Harry D. Smith, Jr. disclosed alogging system wherein earth formation is irradiated with fast neutrons.A single fast neutron detector is used to measure a resulting neutronenergy spectrum by an unfolding process. The patent does not disclosespecific detector type, and whether or not gamma ray discrimination isachieved.

[0023] U.S. Pat. No. 4,631,405 to Harry D. Smith, Jr. discloses a dualspaced fast/epithermal neutron porosity logging system. Fast neutronsare measured at a short spacing with respect to a fast neutron source,and epithermal neutrons are measured at a long spacing with respect tothe neutron source. Measurements are combined to obtain formationporosity.

[0024] U.S. Pat. No. 5,068,532 to Malcolm R. Wormald et al discloses asystem wherein fast neutrons are detected for the purpose of providingcoincident-timing information in lieu of using a pulsed neutron source.The detector is not used to produce borehole logging information,although logging is mentioned in one application.

[0025] U.S. Pat. No. 5,008,067 to John B. Czirr discloses a method formonitoring the output of fast neutrons from a neutron source element ofa well logging apparatus. The detector comprises a scintillatorcontaining oxygen. The ¹⁶O(n,p)¹⁶N reaction induced by 14 MeV neutronsproduces delayed and very large amplitude pulses resulting from the sumof detected beta-decay energy and the 6-7 MeV gamma radiation from thedecay of 16N. These pulses can be separated from other neutron and gammaray pulses.

[0026] U.S. patent application Ser. No. 09/066,729, assigned to theassignee of the present application, discloses a logging system in whichfast neutrons and inelastic scatter gamma rays are measured and combinedto determine formation porosity (and therefore density), and alsocombined to determine formation liquid saturation. A liquid scintillatoris identified for fast neutron detection, providing both fast neutronand inelastic gamma ray counts by pulse shape discrimination. Analternate plastic scintillator and gamma ray detector combination isalso taught in the event that a liquid scintillator is not suitable fora particular application. Fast neutron energies are distinguished by useof pulse height discrimination to provide borehole size compensation forair filled boreholes.

[0027] In view of the above discussion of prior art, it is apparent thatan improved detector for directly measuring fast neutron fluxes in harshborehole environments is needed. Furthermore, it is apparent that a fastneutron detector with efficiency maximized for a given detector geometryis also needed. This disclosure addresses both of these needs.

SUMMARY OF THE INVENTION

[0028] A geometrically optimized fast neutron detector is fabricated ofalternating regions of non scintillating, hydrogenous, opticallytransparent material and scintillation material. The interfaces betweenalternating regions are critical to the detector's fast neutronresponse. One geometry comprises alternating, concentric, rightcylinders of activated ZnS scintillator material and non scintillatingplastic. The ZnS activator can be Ag or Cu or any other suitableactivator. Again, the symbol ZnS is used to denote activated zincsulfide, which can be activated with a variety of dopants. The detector,however, is not limited to cylindrical geometry and may utilizealternate types of scintillator material. The plastic denotes a materialthat is rich in hydrogen (H) and that is optically transparent. Fastneutrons interact with the plastic producing recoil protons which enterthe ZnS scintillation material. The ZnS material is normally potted witha binder such as epoxy, which also contains H. Therefore, some protonrecoils will also occur within the ZnS scintillator region. Protonscreate scintillations within the ZnS, and a portion of this lightescapes the ZnS, enters the transparent plastic, and is detected by aphotomultiplier (PM) tube which is optically coupled to the detector.The PM dynode string is electrically connected to pulse amplificationcircuitry. Recoil proton energy is a function of fast neutron energyimpinging upon the plastic component of the detector. The intensity ofthe scintillation is a function the energy of recoil protons enteringthe ZnS scintillation material. The amplitude of the pulse from theamplification circuitry of the PM tube is a function of the intensity ofthe scintillation. The number of output pulses is a measure of fastneutron flux, and the amplitude of the pulses is a measure of fastneutron energy. Pulse amplitude is also affected by the position atwhich the proton recoil reaction occurs within the plastic material.This effect must be considered in using the detector in fast neutronspectrometry systems, as will be discussed in more detail in asubsequent section of this disclosure.

[0029] Recoil protons have a limited range within the plastic materials.Only proton recoil events occurring near a plastic-ZnS interface willenter the scintillation material and therefore create a scintillation.ZnS is not light transparent. As a result, only proton scintillationevents occurring near a ZnS-plastic interface enter a transparentplastic cylindrical annuli, and are eventually detected by the PM tubeand recorded as a fast neutron event.

[0030] There is also evidence that additional proton flux is generatedwithin the ZnS scintillation material by fast neutrons through the³²S(n,p)³²P reaction. These protons also create scintillations withinthe ZnS material.

[0031] For a given overall detector diameter D, efficiency can generallybe increased by decreasing the radial wall thickness of the ZnS andplastic cylinders, thereby increasing the ZnS-plastic surface area. If,however, the wall thickness of the plastic cylinders is decreased toomuch, the cylinders cease to become an efficient source of recoilprotons, and further cease to become a scintillation “light path” to thePM photocathode. Furthermore, if the radial wall thickness of the ZnScylinders is decreased too much, the cylinders will not scintillate allentering recoil protons. Stated another way, the ZnS and plasticcylinder wall thicknesses for maximum detector efficiency is a“trade-off”, and these dimensions must be optimized for a given detectordiameter D. Detector efficiency can also be increased by increasing thelength L of the detector. Length is also a trade-off parameter in thatexcessive length can decrease the detector's gamma ray rejectioncapability, and further decrease the efficiency of the plastic annuli aslight paths to the PM photocathode.

[0032] The geometrically optimized ZnS/plastic fast neutron detector isideally suited for use in any downhole instrument in which a measure offast neutrons is desired. One application is disclosed in the previouslyreferenced U.S. patent application Ser. No. 09/066,729, assigned to theassignee of the present application, and hereby incorporated in thisdisclosure by reference. The logging system uses measures of fastneutrons and inelastic scatter gamma rays, which are combined todetermine formation porosity (and therefore density), and also combinedto determine formation liquid saturation. A pulsed neutron generatorprovides a source of fast neutrons. Sodium iodide is a suitableinelastic gamma ray detector, wherein the detector is wrapped with athermal neutron absorbing material such as cadmium to prevent neutronactivation of the crystal. A ZnS/plastic detector is used to measurefast neutrons, wherein the geometry of the detector is optimized formaximum efficiency for the space available for the detector within theinstrument or logging “tool”. A ratio of fast neutron energies isdetermined by use of pulse height discrimination to provide boreholesize compensation for gas filled boreholes. As mentioned previously,both impinging fast neutron energy and the position at which a neutroninduced proton recoil event occurs within the plastic component of thedetector affect measured pulse amplitude. It is necessary to account forproton energy loss, commonly referred to as “dE/dx”, as the proton movesfrom the plastic component into the scintillation component, as will bediscussed subsequently.

[0033] The geometrically optimized detector is suited for use as amonitor of output from fast neutron sources. This application is notonly applicable to well logging apparatus and methods, but alsoapplicable to a wide range of analytical and testing methods andapparatus which use fast neutron sources.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] So that the manner in which the above recited features,advantages and objects of the present invention are attained can beunderstood in detail, a more particular description of the invention,briefly summarized above, may be had by reference to the embodimentsthereof which are illustrated in the appended drawings. It is to benoted, however, that the appended drawings illustrate only typicalembodiments of the invention and are therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

[0035]FIG. 1a is a cross sectional view of a prior art ZnS/plastic fastneutron detector;

[0036]FIG. 1b is a side sectional view of a prior art ZnS/plastic fastneutron detector;

[0037]FIG. 2a is a plot of pulse height from a ZnS/plastic fast neutrondetector as a function of time, wherein the pulses are induced byreactions from a pulse of 14 MeV neutrons;

[0038]FIG. 2b is a plot of 14 MeV neutron output from a neutrongenerator as a function of time;

[0039]FIG. 3a is a conceptual illustration of the effects of ZnS-plasticsurface contact area upon the efficiency of a ZnS/plastic fast neutrondetector;

[0040]FIG. 3b is a conceptual illustration of the effects of wallthickness of alternating, concentric plastic cylinders upon theefficiency of a ZnS/plastic fast neutron detector;

[0041]FIG. 3c is a conceptual illustration of the effects of detectorlength upon the efficiency of a ZnS/plastic fast neutron detector;

[0042]FIG. 4a is a cross sectional view of a concentric, coaxial,geometrically optimized ZnS/plastic fast neutron detector;

[0043]FIG. 4b is a side sectional view of a concentric, coaxial,geometrically optimized ZnS/plastic fast neutron detector;

[0044]FIG. 4c is a perspective view of an axially layered, geometricallyoptimized ZnS/plastic fast neutron detector;

[0045]FIG. 4d is a perspective view of an axial bar, geometricallyoptimized ZnS/plastic fast neutron detector;

[0046]FIG. 4e is a perspective view of an axial rod, geometricallyoptimized ZnS/plastic fast neutron detector;

[0047]FIG. 5 is an illustration of a geometrically optimized ZnS/plasticfast neutron detector embodied in a nuclear well logging system;

[0048]FIG. 6 illustrates amplification, gain control and gamma rayrejection discriminator components of the electronics section of thewell logging system;

[0049]FIG. 7 is a conceptual illustration of directional detectorsensitivity of the geometrically optimized detector; and

[0050]FIG. 8 is a functional diagram of a fast neutron monitoring systemusing the geometrically optimized detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] A fast neutron detector suitable for use in borehole logginginstruments must detect neutrons efficiently in the energy ranges fromabout 1 million electron volts (MeV) to 14 MeV, reject any response toaccompanying radiation, be mechanically rugged, and operate at atemperature of about 175° C. The combination of separate regions ofactivated ZnS scintillator and hydrogen rich plastic produces a suitablefast neutron detector when optically coupled to a photomultiplier (PM)tube. Energetic protons are produced by fast neutrons scattering fromhydrogen within the plastic. A scintillation photon is produced when anenergetic proton reaches the ZnS scintillator. Because the linear rangeof energetic protons is very short in both the scintillator and plasticmaterials, and because light must escape the optically cloudy ZnS regionto be detected by the PM tube, the interface between the two materialsis critical to the detector's fast neutron response. Therefore, thetotal surface area of the ZnS-plastic interface is an important designconsideration. Other important design considerations are the volume ofthe detector, the detector diameter and length, the geometric shapes ofthe regions of scintillator and plastic material, the operatingtemperature that should be at least 175° C., and the manufacturabilityof the most efficient geometric configuration of the detector.

[0052] Detectors consisting of a combination of ZnS and plastic regionshave shown excellent gamma radiation rejection on the basis of pulseheight. That is, the PM tube pulses produced by fast neutrons in thedesired energy range are much larger in amplitude than pulses induced bygamma radiation with similar energy.

[0053] As will be illustrated in a subsequent section of thisdisclosure, gamma ray induced pulses are sufficiently small in amplitudeso that a discriminator circuit can be set to reject gamma ray pulseswith little loss in fast neutron response. ZnS alone has a measurablefast neutron response. However, when used alone, ZnS is restricted torelatively small volumes due to lack of light transparency and theresulting inability of scintillation photons occurring within thematerial to reach the photocathode of an optically coupledphotomultiplier (PM) tube. Larger volume, more efficient scintillationdetectors can be obtained by combining ZnS with a nonscintillating,optically transparent hydrogen rich plastic. The plastic producesadditional fast neutron response and provides an optical path forscintillation photons to the photocathode of the optically coupled PMtube.

[0054] It should be understood that various scintillation materials canbe used in combination with the plastic component. As an example, thezinc sulfide dopant can be Ag or Cu depending upon temperaturerequirements, physical geometry constraints, and even economiclimitations. The plastic could be replaced with another hydrogen richand optically transparent material, such as RTV.

Prior Art Detectors

[0055] A prior art detector is shown as a cross sectional view in FIG.1a and as a side sectional view in FIG. 1b. The detector is identifiedas a whole by the numeral 50. Geometrically, the detector is constructedwith a ZnS cylindrical core or “bulls eye” 58′ surrounded by alternatingand concentric cylinders 60 of plastic and cylinders 58 of ZnS. The core58′ and alternating concentric cylinders 58 and 60 are bound together asa unit preferably by epoxy and enclosed at one end and around theperiphery within a an opaque cover 52. The second, uncovered end isoptically coupled at interface 51 to a PM tube 54 as best seen in FIG.1b.

[0056] Detectors of the type and geometry shown in FIGS. 1a and 1 b aremade commercially by the Bicron division of Saint-Gobain InternationalCeramics, Inc. More specifically, the Bicron model BC-720 scintillator'slength “L” dimension 62 is 0.625 inches (in.) or 15.88 millimeters (mm),and the diameter “D” dimension 64 1.5 inches (in.) or 38.1 millimeters(mm). Other diameters are currently available, but only with a length of0.625 in. There are two concentric ZnS cylinders and three plasticcylinders 60 arranged alternately around the ZnS core 58′. Allcomponents of the scintillator are potted in epoxy. The wall thicknesses66 and 68 of all plastic cylinders 60 and ZnS cylinders 58 are both0.125 in. (3.175 mm) in the model BC-720 scintillator. The plastic ismilled with concentric recesses. A bottom base plate 59 contacts the PMtube 54 at the interface 59. The concentric cylinders 58 of ZnS arefitted into the concentric recesses with an epoxy mixture.

Principles of Operation

[0057] As mentioned previously, the ZnS/plastic detector assemblyresponds through the mechanism of fast neutrons interacting withhydrogen within the plastic cylinders to produce recoil protons. Recoilprotons enter the ZnS cylinders. Protons create scintillations withinthe ZnS cylinders. A portion of light produced by the scintillationsescapes the ZnS and enters the transparent plastic cylinders and issubsequently detected by a photomultiplier (PM) tube that is opticallycoupled to the detector. The PM photocathode is electrically connectedthrough a dynode string to pulse amplification circuitry. Recoil protonenergy is a function of fast neutron energy impinging upon the plasticcomponent of the detector. The intensity of the scintillation is afunction of the energy of recoil protons entering the ZnS scintillationmaterial and also depends on the dE/dx of protons within the plastic.The amplitude of the pulse from the amplification circuitry of the PMtube is a function of the intensity of the scintillation. The number ofoutput pulses is a measure of fast neutron flux, and the amplitude ofthe pulses is a measure of fast neutron energy.

[0058]FIGS. 2a and 2 b illustrate the response of the fast neutrondetector resulting from irradiating earth formation or other materialwith several bursts or “pulses” of fast (14 MeV) neutrons. The responseshown is a superposition of many such bursts.

[0059]FIG. 2b is a plot of neutron output (ordinate) from a fast neutronsource as a function of time T (abscissa). The burst is initiated at atime T₁ as indicated by the point 70 on the time axis, and terminated ata time T₂ as indicated by the point 71 on the time axis.

[0060]FIG. 2a shows the corresponding output from the photomultipliertube and amplification circuit of the detector assembly. Pulse height oramplitude (ordinate) is plotted as a function of time T (abscissa).Prior to the burst initiation at T₁ there are essentially no outputpulses. Once the burst is initiated at T₁, relatively high amplitudepulses 72 are observed as a result of recoil protons inducingscintillations within the ZnS component of the detector. The pulseamplitude is a function of recoil proton energy which, in turn, is afunction of fast neutron energy impinging upon the detector. The numberof high amplitude pulses is a function of the intensity of fast neutronflux impinging upon the detector. High amplitude pulses cease to beobserved at time T₂ when the neutron burst is terminated. Smaller pulses74 are also observed during and after termination of the neutron burst.These result primarily from gamma radiation produced by the capture ofthermal neutrons after the burst. It is apparent that this gammaradiation generates pulses 74 of a much lower amplitude than pulsesgenerated by the fast neutron-recoil proton process. By setting adiscriminator level at a pulse height in excess of the amplitude forgamma radiation, for example 76, and recording pulses of amplitudegreater than this level, gamma radiation can be effectively removed fromthe detector response. The detector then responds only to fast neutronsover the range of about 0.5 to 14 MeV. The detector also exhibits noise,which is lower in amplitude than the neutron induced pulses 72. Noiserelated pulses are not shown in FIG. 2a for purposes of clarity.

[0061] As stated previously, there is also evidence that additionalproton flux is generated within the ZnS scintillation material by fastneutrons through the ³²S(n,p)³²P reaction. These protons also createscintillations within the ZnS material. Proton recoils may also beproduced in the scintillation material if the ZnS binder contains H.

Geometric Optimization of Detector Efficiency

[0062] The process of optimizing the ZnS-plastic combination for thedetection of fast neutrons is presented by way of example. The exampleuses the geometry of the prior art detector, consisting of alternatingconcentric cylinders of ZnS and plastic. This geometry is shown in FIGS.1a and 1 b.

[0063] Recoil protons have a limited range within the plastic material.Only proton recoil events occurring near a ZnS-plastic interface willenter the scintillation material and therefore create a scintillation.As a specific example, consider the use of an acrylic resin such asLucite for the plastic cylinder components of the detector. Lucite meetsthe requirements of being a hydrogen rich source of recoil protons, isoptically transparent, and is suitable for moderate temperaturesencountered within a borehole environment. The range of a 5 MeV protonis 0.04 grams per square centimeter (g/cm²). For the density of 1.19grams per cubic centimeter (g/cm³) for Lucite, the linear range of the 5MeV proton is only 0.033 cm. Recall that the wall thickness of all ofthe plastic cylinders of the Bicron model BC-720 scintillator assemblyis 0.3175 cm. It is apparent, therefore, that a large portion of recoilprotons created by fast neutron reactions in the interior of the plasticcylinders have insufficient linear range to reach the nearest ZnSscintillator, create a scintillation, and therefore be detected. Statedanother way, excess plastic cylinder thickness decreases the overallefficiency of the detector assembly to fast neutrons for a givendiameter D. Efficiency can be optimized by varying the plastic cylinderwall thickness and varying the number of cylinders, but for a givenoverall detector diameter D.

[0064] ZnS is not light transparent. As a result, only protonscintillation events occurring near a ZnS-plastic interface enter atransparent plastic cylinder and are subsequently detected by the PMtube and recorded as a fast neutron event. As in the case of the plasticcylinders, excess ZnS cylinder wall thickness can decrease the overallefficiency of the detector to fast neutrons, and efficiency can beoptimized by optimizing ZnS cylinder wall thickness by using more butthinner cylinders for a given overall detector diameter D.

[0065] There are, however, “opposing” factors that affect detectoroptimization. Recall that the plastic cylinders serve not only as asource of hydrogen for the proton recoil reaction, but also serve aslight paths through which scintillations pass from the detector assemblyto the optically coupled PM tube. If the wall thickness of the plasticcylinders is reduced excessively, the transmission of scintillationlight is impaired.

[0066] Length of the detector can also be increased to conceptuallyincrease detector efficiency. Applications of the detector may limit thelength. In addition, increasing length L can also increase the ratio ofgamma radiation pulse amplitude to the proton recoil pulse amplitudethereby making the rejection of gamma radiation events by pulse heightdiscrimination more difficult. Once again, increasing efficiency by thistechnique presents a trade-off.

[0067] As mentioned previously, the dE/dx effect of proton transportthrough the plastic must be considered if the detector is to be used asa fast neutron spectrometer. Conceptually, average dE/dx is reduced asthe thickness 66 of the plastic cylinders are decreased. This may resultin additional alternating cylinders of plastic and ZnS for a givendetector diameter D. Such an optimization for spectroscopy reasons mightdecrease the overall efficiency of the detector. Stated another way, ifthe detector is to be used as a fast neutron spectrometer, optimizationmust include both efficiency and spectroscopy considerations.

[0068] There is also evidence that additional proton flux is generatedwithin the ZnS scintillation material by fast neutrons through the³²S(n,p)³²P reaction. These protons also create scintillations withinthe ZnS material, and the wall thickness of the cylinders determines theportion of scintillation reaching the plastic cylinder light paths.

[0069] The factors governing geometric optimization of a ZnS/plasticscintillator of given diameter D and length L are shown conceptually inFIGS. 3a, 3 b and 3 c.

[0070]FIG. 3a is a plot of an efficiency term E(SA) as a function ofsurface contact area SA between the ZnS and plastic cylinders. Thiscontact area is, of course, governed by the number n of alternating ZnSand plastic cylinders. For a given diameter D, SA is a function of thethickness tsi of the ZnS cylinders, where (i=1, . . . , n). For reasonsdiscussed above, E(SA) increases with increasing SA as is illustrated bythe curve 80.

[0071]FIG. 3b is a plot of an efficiency term E(1/t_(pi)) as a functionof the inverse of plastic cylinder wall thickness t_(pi), where again(i=1, . . . , n). For a given value of D and for reasons discussedabove, E(1/t_(pi)) varies as a function of 1/t_(pi) as is illustrated bythe curve 81. E(1/t_(pi)) increases as a function of 1/t_(pi) to acertain value 83, and then starts to decrease as increasingly thinplastic cylinder walls impede the passage of scintillation light to thePM tube.54 (see FIG. 1b).

[0072]FIG. 3c is a plot of an efficiency term E(L) as a function ofdetector length L. For a given value of D, E(L) increases as a functionof L for reasons discussed above. At some value of L, further increasein length does not increase E(L) since scintillation light, formed inthe portion of the detector opposite the PM tube, can not reach the PMtube and there be sensed. This point is conceptually illustrated at 84.

[0073] The total detector efficiency, E_(total), for a given detectordiameter D, can be expressed mathematically as

E _(total) =f(E(SA),E(1/t _(pi)),E(L),E(A _(γ)))  (1)

[0074] where f(E(SA),E(1/t_(pi)),E(L), E(A_(γ))) is a functioncontaining the efficiency terms E(SA), E(1/t_(pi)), and E(L). E(A_(γ))is an efficiency term which accounts for the relative amplitude of gammaray induced pulses as a function of detector length. For a givendetector diameter D, which is usually determined by the physicalapplication of the detector in an instrument such as a logging tool, theparameters ZnS cylinder wall thickness tsi (and thus SA), plasticcylinder wall thickness (and thus1/t_(pi)), and length L are adjusted tomaximize the total counter efficiency E_(total). If the physicalapplication also restricts the length L, then only the first twoparameters are adjusted to maximize E_(total). E(A_(γ)) is not anadjustable term. The efficiency of a ZnS/plastic detector can,therefore, be customized for a given physical dimension to geometricallyoptimize the detection efficiency for fast neutrons.

[0075] The detector can also be optimized for fast spectroscopyapplications by minimizing the previously discussed dE/dx energy loss ofrecoil protons within the plastic component of the detector. Anotherefficiency term E(dE/dx) expresses the functional dependency of theenergy loss dE/dx. The detector efficiency for fast neutronspectroscopy, Espec, for a given detector diameter D, can be expressedmathematically as

E _(spec) =f(E(SA),E(1/t _(pi)),E(L),E(A _(γ)),E(dE/dx))  (2)

[0076] The efficiency terms E(SA), E(1/t_(pi)), E(L) and E(dE/dx) areadjusted to maximize E_(spec). It should be noted that E(dE/dx) is alsoa function of the plastic thickness tpi for reasons discussed above.

[0077]FIGS. 4a and 4 b illustrate end and side sectional views,respectively, of a geometrically optimized ZnS/plastic fast neutrondetector 100 comprising n pairs of ZnS and plastic cylinders. The axialcenter line of the detector is identified by the numeral 92. The ZnScylinders 88 are denoted as si (i=1. . . . , n) starting with the corecylinder 88′ as s₁. The alternating plastic cylinders are denoted asp_(i) (i=1. . . . , n). Cylinder wall thicknesses for the ZnS cylinders88 and plastic cylinders 90 are identified as t_(si) and t_(pi),respectively, where (i=1, . . . , n). It should be noted that cylinderwall thicknesses need not be equal. That is, it is not necessary fort_(pj) to equal t_(pj+1), or t_(sj) to equal t_(sj+1), or t_(pj) toequal t_(pj), where 1<j<n. As an example, it might be advantageous tofabricate a detector where t_(si)<t_(pi) for any or all values of i=1. .. n. Cylinder thicknesses can be adjusted in any manner to maximize thetotal efficiency E_(total), or the spectroscopy efficiency E_(spec), aslong as the constraints on D (identified as dimension 84) and L(identified as 86) of the detector 100 are satisfied.

[0078] The detector is encapsulated within a light-tight housing (notshown) with the exception of the end that is optically coupled to the PMtube.

[0079] It should be noted that materials used in a geometricallyoptimized fast neutron detector are not limited to Lucite and ZnS. Theplastic cylinders can be fabricated from any material which provides ahydrogen rich source for the proton recoil reaction, which is opticallytransparent, and which can withstand high temperatures encountered inthe borehole environment. The scintillating cylinders can be fabricatedfrom any material which scintillates upon proton bombardment, and whichproduces scintillations of intensity related to the energy of thebombarding protons.

[0080] It should also be noted that the geometrically optimized detectoris not limited to a right cylindrical shape. As an example, alternatingrectangular layers of plastic and scintillating material can be used tofabricate a detector. FIG. 4b could equally represent a cross sectionalview of a rectangular detector.

[0081] Optimization methods previously described for the right,concentric cylindrical detector are also applicable to a rectangulardetector, and to other geometric forms forming a right cylinder. FIG. 4cis a perspective view of a layered detector 300 comprising alternatingregions of scintillation material 302 and plastic material 304 in theform of panels. FIG. 4d is a perspective view of an axial bar detector320 comprising a grid of perpendicular layers of scintillation material322 contacting plastic material 324. FIG. 4e is a perspective view ofaxial rod detector 340 shown with a cut-away to more fully discloseconstruction. Cylinders of scintillation material 342 are dispersedaxially within a surrounding right axial cylinder of plastic material344 and, in the example shown, extend from the top to the bottom of thecylinder of plastic material. Conversely, in FIG. 4e the dispersedcylinders 342 could be plastic material and the cylinder 344 thescintillation material. Cylinders 342′ are shown in the cut-away portionof the cylinder 344 for purposes of illustration.

[0082] Detectors can also be fabricated in conic, triangular, orvirtually any shape required for a particular application. The onlyrequirement for operation and optimization set forth in this disclosureare (a) that the detector comprise alternating components of plastic andscintillator material, and (b) that the plastic component provides apath for scintillation light to a PM tube or an alternate device whichresponds to impinging light.

Logging Application of an Optimized ZnS/Plastic Detector

[0083]FIG. 5 depicts a logging tool 21 suspended within a borehole 40 bya logging cable 24. The borehole 40 penetrates earth formation 32, andis shown cased with typically steel casing 38. The annulus defined bythe outside diameter of the casing 38 and the borehole wall 34 is filledwith cement 36. One end of the logging cable 24 is mechanically andelectrically connected to the logging tool 21 by means of a cable head23. The cable 24 then passes over an upper sheave wheel 26′ and acalibrated sheave wheel 26 and to a winch reel 31. A depth indicator 27is attached to the sheave wheel 26 thereby permitting a measurement ofthe amount of cable deployed in the borehole 24, and therefore the depthof the logging tool 21 in the borehole 40. The depth indicator iselectrically connected to a set of surface equipment 28 which is used tocontrol the operation of the logging tool 21 and to process and storedata measured by the logging tool. The surface end of the logging cableterminates in a winch 31, and is electrically connected to the surfaceequipment 28 through electrical slip rings 29. Measurements made by thelogging tool 21 are output to a recorder 30 which forms a tabulation ofthe measurements as a function of depth at which they were measuredthereby creating the previously defined “log” 33. It should beunderstood that the log 33 can be in the form of an analog plot, adigital tabulation, or even a magnetic recording such as a tape or disk.

[0084] While FIG. 5 depicts a logging tool, the optimized ZnS detectorcould likewise be used in a logging-while-drilling tool of the typeillustrated in U.S. Pat. No. 5,091,644, and conveyed within the borehole40 by means of a drill string.

[0085] Still referring to FIG. 5, the logging tool 21 comprises apressure tight housing 22 which contains a neutron generator assembly20, a “near” axially spaced gamma ray detector 14 and a “far” axiallyspaced gamma ray detector 14′ such as sodium iodide, and a “far” axiallyspaced, geometrically optimized ZnS/plastic fast neutron detector 100biased to detect fast neutrons over a range of about 0.5 MeV to 14 MeV.The neutron generator assembly 20 comprises a high energy neutronproducing tritiated target 10 with associated deuterium gas reservoirand accelerated ion beam, high voltage supplies and pulsing electronics(not shown). Neutron generators, which produce 14 MeV neutrons by thedeuterium-tritium reaction, are well known in the art and arecommercially available.

[0086] A monitor detector 42 is preferably located close to the neutrontarget 10 in order to monitor the somewhat variable neutron outputproduced by this type of generator. The monitor detector is also ageometrically optimized ZnS/plastic fast neutron detector. The monitordetector bias is set to measure neutrons of energy of about 12 MeV orhigher in order to more closely monitor the direct fast neutron outputfrom the neutron generator 20 and reject events from neutrons that havebeen scattered by the surrounding environs. Logging system response canbe normalized to a fixed or “standard” neutron output by means of themonitor detector response. The use of the optimized detector as a fastneutron monitor in other applications will be discussed in detail in asubsequent section of this disclosure.

[0087] The neutron source is pulsed and is capable of variations in boththe pulse repetition rate and the pulse width. The preferred embodimentfor a formation porosity/gas saturation logging system employs a pulserepetition rate of 3,000 Hz with a pulse width of about 30 μsec.

[0088] Still referring to FIG. 5, a fast neutron and gamma ray shield 12is placed between neutron generator target 10 and the near gammadetector 14. The shielding material is preferably a material such assteel which will scatter, rather than thermalize, fast neutrons emittedfrom the target 10 into the formation 32 so that they can interact withformation nuclei thereby providing useful parametric information. Theshield 12 also preferably contains an effective gamma ray shieldingcomponent to shield the gamma ray detector 14 from gamma radiationinduced by the emitted neutron flux in the immediate vicinity of thetarget 10. The near detector 14 is located as close as possible to thetarget 10, while allowing for an adequate axial thickness for shield 12.The spacing of the near detector 14 from the target 10 is about 25 cm.The near gamma detector 14 preferably consists of a scintillator coupledto a photomultiplier tube. As stated previously, sodium iodide is anadequate scintillator for the near detector 14, although a scintillatorwith a faster decay time and made of a material that does not activateby thermal neutrons would be preferred. If sodium iodide is used fornear detector 14, then it must be wrapped with a thermal neutron shieldsuch as cadmium to minimize thermal activation within the crystal, andthe gamma ray emissions resulting from the activation. In addition, theenergy response of the detector is preferably electronically biased toexclude activation produced pulses within the detector crystal.

[0089] Again referring to FIG. 5, a fast neutron and gamma shield 16 isplaced between near detector 14 and fast neutron detector 100. Thisshield, like the shield 12, prevents fast neutrons from the neutrongenerator assembly 20, and neutron induced gamma radiation, from“streaming” directly down the axis of the logging tool 21 and into thedetector 100. If sodium iodide is used for far gamma ray detector 14′,it must also be shielded with cadmium.

[0090] An electronics package 18 controls and provides power to thevarious electronic components of the downhole instrument 21. Pulseamplitude counting and biasing circuitry is included within theelectronics package 18 providing the previously discussed bias levelsfor the fast neutron detectors 42 and 100. FIG. 6 illustrates some ofthe components included in the electronics package 18. Pulses from thePM tube 54 are amplified by an amplification circuit 150. Amplifiedpulses then pass through a gain control circuit 152, and subsequentlythrough a discriminator 154 in which pulses resulting from gammaradiation impinging upon the detector 50 are rejected.

[0091] Attention is next directed toward the analysis of the response ofthe fast neutron detector 100 to fast neutrons, and the gamma raydetectors 14 and 14′ to both prompt and delayed gamma radiationresulting from inelastic scatter and thermal capture reactions,respectively. Detectors 14, 14′ and 100 in FIG. 5 are gated so thattheir pulses are processed and stored during certain time intervals inreference to the beginning of each pulse of neutrons from the neutrongenerator 20. These time gates are designed to produce detectorresponses to both prompt and to delayed radiations. The fast neutrondetector 100 is gated ON during the neutron burst between times T₁ andT₂ as illustrated in FIGS. 2a and 2 b. The gamma ray detectors are gatedON also during the time interval T₁, to T₂, during which both promptgamma radiation and capture gamma radiation are detected. The gamma raydetectors 14 and 14′ are subsequently gated ON for two additional timeintervals after time T₂, which is the termination of the neutron burst.These two additional gamma ray detector gates are used to measurethermal capture radiation, which is subsequently subtracted from thegamma ray count rate measured during the burst leaving only a measure ofthe desired prompt gamma radiation in the time interval T₁ to T₂. A moredetailed description of this process is found in the previouslyreferenced U.S. patent application Ser. No. 09/066,729.

[0092] The porosity (or density) and water or gas saturation of earthformations 32 are determined by combining measures of prompt gammaradiation made with detectors 14 and 14′ resulting from fast neutronreactions, and measures of fast neutron radiation made with detector100. Interpretation charts determined from calibration measurements withthe logging system in known formation and borehole conditions, and fromcomputer simulations, are used to infer the formation parameters ofinterest (porosity and water or gas saturation) from the measured gammaand neutron count rates. These procedures are disclosed in detail in thepreviously referenced U.S. patent application Ser. No. 09/066,729. Theinterpretation is influenced by formation matrix composition and byborehole conditions, and will be uncertain to the extent that theseother parameters are not known. The effects of a gas filled boreholeupon measures of desired parameters of interest can be minimized with ameasure of fast neutrons at a high energy level and a low energy level.Referring to FIG. 2a, pulses falling within the amplitude range frompoint 76 to point 77 represent fast neutrons at a low energy level.Pulses falling within the amplitude range from point 77 to 79 representfast neutrons at a high energy level. Combining the measures of high andlow energy fast neutrons yields a borehole correction. Corrections tominimize the effects of variations in liquid and gas filled boreholeconditions are disclosed in detail the previously referenced U.S. patentapplication Ser. No. 09/066,729.

[0093] Referring again to FIG. 5, the high energy neutron source 20repetitively emits pulses or bursts of neutrons at target 10 inrelatively short duration of approximately 30 μs, with a repetition rateof about 3,000 pulses per second. The fast neutrons from target 10 reactpromptly with atomic nuclei in formation 32, principally silicon andoxygen in the case of a silica matrix, or calcium, carbon and oxygen inthe case of a limestone matrix. Some of the fast neutrons also scatterfrom the hydrogen associated with any water present in the formation. Ofcourse, the neutrons also react with nuclei in the tool housing 22 andnearby borehole casing 38, cement 36, and any liquid that might be inthe borehole 40. These latter reactions produce undesirable responses inthat they yield no information concerning the formation parameters ofinterest and further interfere with the response of interest from theformation. As discussed previously, detectors 14, 14′ and 100 are gatedon during the time interval T₁, to T₂ to detect fast neutrons and promptgamma rays during the time of the neutron burst. The prompt gammaresponse contains, as discussed earlier, an unwanted capture componentdue to the thermalization of fast neutrons and their subsequent captureduring the time of the neutron source pulse. Corrections are made toeliminate the thermal capture components.

[0094] Again referring to FIG. 5, some of the gamma rays detectedpromptly during the neutron burst, by detectors 14, 14′ and 100, arefrom inelastic scattering of the fast neutrons from nuclei in formation32. Some of the fast neutrons detected by fast neutron detector 100 havescattered elastically and inelastically from nuclei in formation 32. Thedetected neutrons have an energy in the range from 0.5 to 14 MeV.Scattering from the hydrogen contained in the formation 32 has a uniqueeffect on the flux and the energy distribution of fast neutrons in thevicinity of the gamma ray detectors 14 and 14′, and fast neutrondetector 100. This is due to two properties of hydrogen which are (1)the scattering probability is substantial at the 0.5 MeV minimumdetected energy, but decreases rapidly in the detected range from 2 to14 MeV, and (2) the hydrogen nucleus has nearly the same mass as theneutron so that, unlike any other element, it can acquire most or allthe energy from a fast neutron in a single scattering. The fast neutronmeasurement therefore has a special response to the hydrogen associatedwith moisture or hydrogen content of the formation surrounding theborehole. For most earth formations, the rock formation does not containhydrogen, and this special response is associated uniquely with theliquid that fills pore space of the rock matrix. The combined effects ofthe formation matrix and pore liquid on fast neutron flux lead to asmall dependence on bulk density and a much larger dependence on atomdensity. For partial or no liquid saturation of the pore space, atomdensity decreases and there can be a large change in the neutron flux.If the capture gamma ray component is removed from the prompt gamma raymeasurement, the resulting inelastic gamma ray responses from detectors14 and 14′ are primarily sensitive to changes in bulk density of theformation, which is also a weak function of the moisture content of theformation. Stated simply, the inelastic gamma and fast neutron responsesare both affected by changes in formation density (or porosity) and bychanges in formation water (or gas) saturation. However, the tworesponses depend differently on changes in these formation parameters.Therefore, for a known formation matrix such as silica or limestone, thedisclosed logging system can produce values for water (or gas)saturation and porosity (or bulk density) from the combined measuresfast neutron and inelastic gamma responses by use of a two-dimensionaltransform grid determined from calibration data with known physicalmodels and from computer simulations. A given transform grid isappropriate only for a particular rock matrix and set of boreholeconditions. This transform is discussed in more detail in previouslyreferenced U.S. patent application Ser. No. 09/066,729.

[0095] The logging system utilizing the geometrically optimized fastneutron detector 100 can be effectively operated in cased and openboreholes, and in gas filled and liquid filled boreholes. This does not,however, imply that the measurements are independent of boreholeconditions. The effects of borehole conditions on the measurements can,however, be determined and the measurements can be compensated orcorrected for these borehole conditions as discussed in detail in thepreviously referenced U.S. patent application Ser. No. 09/066,729. Thetransform grids disclosed in U.S. patent application Ser. No. 09/066,729produce the desired formation parameters of porosity (or bulk density)and gas saturation (or water saturation) when the system logs a liquidfilled borehole and measures fast neutron counts versus depth, andinelastic gamma ray counts versus depth, and when the formation mineral(matrix) and borehole conditions conform to those assumed for thenominal transform grid. Stated another way, one transform gridrepresents the response of the logging system for one set of boreholeconditions and for one formation lithology. For gas filled boreholes, atransform grid representing one borehole condition can be used toprocess data measured under different boreholes by combining high energyand low energy fast neutron measurements as previously discussed.

[0096] The use of the optimized fast neutron detector in a specificwireline logging tool has been disclosed in detail above. It should beunderstood, however, that the optimized fast neutron detector can beused in any wireline, measurement-while-drilling, or any other type oflogging tool requiring a measurement of fast neutron flux.

Application of an Optimized Plastic Detector as a Fast Neutron Monitor

[0097] As discussed previously, the disclosed logging system requires ameasure of neutron generator output in order to obtain accurate measuresof parameters of interest. Many non-logging analysis and testing systemsalso use sources of fast neutrons. These systems include activationanalysis systems, mineral assay systems, industrial waste monitoringsystems and the like. Two of these systems will be discussed briefly asbackground information in the disclosure of the geometrically optimizeddetector embodied in a fast neutron monitoring system.

[0098] Fast neutron activation analysis was developed in the late 1950s.Samples of material are irradiated with fast neutrons for apredetermined length of time. Fast neutrons interact with elementswithin the system to form radioactive isotopes. These isotopessubsequently emit delayed gamma radiation of characteristic energy. Withproper calibration of the system, and with proper control of variableparameters such as irradiation time, sample geometry, detectorefficiency and the like, the measured intensity of the characteristicgamma radiation can be related to a concentration of the element ofinterest within the sample. Characteristic gamma ray intensity can alsovary as a function of the output of the fast neutron source. Neutronoutput must, therefore, be accurately monitored in order to obtaindesired parameters of interest.

[0099] A method for analyzing the grade of coal was introduced in the1970s, wherein coal is irradiated with 14 MeV neutrons, and inducedprompt gamma radiation is measured to determine carbon and oxygencontent of the coal. A measure of these elemental concentrations is thenrelated to the quality or “grade” of coal. The intensity of measuredgamma radiation is used to determine element content. Intensity ofmeasured gamma radiation can, however, also vary with variations in theoutput of the neutron generator. An accurate monitor of fast neutronoutput from the source is, therefore, required to obtain accuratemeasures of coal grade.

[0100] Fast neutron sources, operated in an environ other than a vacuum,produces a wide variety of other types of radiations. These radiationsare induced by the fast neutrons interacting with virtually any materialsurrounding the fast neutron source. Epithermal and thermal neutrons aregenerated by the interaction of fast neutrons with any material in thevicinity of the source. Prompt gamma radiation is generated by theinelastic scatter of fast neutrons with material in the surroundingenvirons. Thermal capture gamma radiation is induced by the capture ofthermalized neutrons by elements in the surrounding environs. Previouslydiscussed activation gamma radiation is generated by interaction offast, epithermal and thermal neutrons with elements within materialssurrounding the fast neutron source. Any fast neutron monitor detectormust, therefore, be insensitive to the varying and often intense“secondary” radiations generated by the neutron source, and respond onlyto fast neutrons emitted by the source. If the surrounding environscontains a high concentration of light elements, fast neutrons cease tobe “fast” after the first interaction with such nuclei. This isparticularly true in wellbore environments where the hydrogenconcentration per unit volume is often relatively high.

[0101] Prior art fast neutron monitor systems are relatively numerousand well known. Stated generically, the systems are typically based uponpulse shape discrimination, gamma ray shielding, detector timing, andcombinations of thereof. All suffer from the inability to discriminatethe effects of induced gamma radiation and the inability to discriminatethe effects of neutrons which have undergone some type of scatterreaction, but still posses relatively high energies in the MeV range.The geometrically optimized fast neutron detector minimized theseproblems found in prior art systems.

[0102]FIG. 8 is a functional diagram illustrating a fast neutron source210 such as a 14 MeV neutron generator. A geometrically optimizeddetector 212, optically coupled to preferably a PM tube 214, is placedin close proximity to the source 210. Pulses from the source areamplified with an amplifier circuit 216, and the gain of the system isstabilized with a gain control circuit 217. Pulses from the gain controlcircuit pass through a discriminator 218 where all “events” which haveundergone any type of reaction with nuclei in the surrounding environsare rejected. The output of the discriminator circuit 218 to a counter220 is, therefore, directly related to the output of the fast neutronsource 220.

[0103] Since the detector 210 is placed within close proximity of thefast neutron source 210, the neutron flux impinging upon the detector istypically quite intense. Maximization of detector total efficiency isusually not necessary. The detector is, therefore, geometricallyoptimized for spectroscopy efficiency as stated mathematically inequation (2), rather than for total efficiency as stated mathematicallyin equation (1). With the detector 212 optimized for spectral precision,the discriminator can be set relatively high. For purposes ofdiscussion, assume that the fast neutron source is a deuterium-tritiumneutron generator that produces neutrons at a nominal 14 MeV level.Neutron output is typically not truly monoenergetic, and the outputenergy is typically an angular distribution centered at approximately 14MeV. Furthermore, single scatter events usually decrease the neutronenergy well below approximately 12 MeV. Considering these twomechanisms, a detector bias set at approximately 12 MeV essentiallyrecords unperturbed fast neutron output. Prior art systems do not havethe gamma ray rejection features, and the ability to be spectrallyoptimized to allow a meaningful bias level to be set with accuracy andprecision at approximately 12 MeV.

Other Applications

[0104] The geometrically optimized fast neutron detector can befabricated to exhibit directional neutron detection characteristics.FIG. 7 is a highly conceptualized illustration of possible directionaldetection bias properties of the geometrically optimized detector. Thedetector 200 is shown in sectional view, and can be either cylindricalor rectangular Alternating layers of plastic 172 and scintillatingmaterial 174 are shown. Assume that a neutron 180 impinges upon thedetector from the right, travels a distance 1 ₁ identified as 190 withinplastic material 172, and interacts at a point 186 to form a recoilproton. Assume that a second neutron 182 impinges upon the detector fromthe right, travels a distance 1 ₂ identified as 192 within plasticmaterial 172, and interacts at a point 188 to form a recoil proton.Finally, assume that a third neutron 184 impinges upon the detector fromthe right, travels a distance 1 ₃ identified as 194 within plasticmaterial 172, and interacts at a point 189 to form a recoil proton.There is a high statistical probability that all three recoil protonswill enter scintillating material 174, create scintillations, and thesescintillations will be sensed by an affixed PM tube (not shown). Itshould be noted that detection probability is relatively high only ifthe proton is produced close to the interface with the scintillatormaterial. Stated simply, the three neutrons 180, 182 and 184 impingingupon the detector from the right will probably be “detected”. Nextconsider the same three neutrons, now denoted by 180′, 182′ and 184′,impinging upon the detector from the top. Assuming for simplicity thatthe fast neutron attenuating properties of plastic and scintillationmaterial are the same, the neutron 180′ will (statistically) travel thedistance l and interact at a point 186′ to form a recoil proton. Again,there is a high statistical probability that neutron 180′ will bedetected. If, however, neutrons 182′ and 184′ travel the samestatistical path lengths 12 and 13, they will pass through the detectorundetected. In summary, there is a high statistical probability thatthree neutrons entering the detector from the right will be detected.Conversely, there is a high statistical probability that only one of thethree neutrons will be detected when impinging upon the detector fromthe top. This is because proton production is “directional”, andscintillation photon production is proportional to proton production.The efficiency of the detector fabricated and oriented as shown in FIG.7 is, therefore, highly biased in the horizontal direction.

[0105] Although the geometrically optimized fast neutron detector wasconceived for use in boreholes to log parameters such as density(porosity) and gas (water) content in subsurface geologic formations,the detector has other applications in apparatus to analyze othermaterials. As an example the ZnS/plastic fast neutron detector can beused to analyze highway roadbeds prior to paving the surface. It is veryimportant to establish that this substrate material is of the properbulk density and moisture content before the paving process begins. Theexisting nuclear gamma density and neutron moisture gauges used for thispurpose are losing favor with the industry because of the requiredisotopic radioactive sources and their related licensing and safetydifficulties. Apparatus of the present invention comprising ageometrically optimized ZnS/plastic fast neutron detector, a promptgamma ray detector, and a pulsed neutron source which can safely bedeactivated represents a desirable replacement for existing density andneutron gauges for the highway surface applications. Similarapplications may exist in industrial process control where materialdensity and/or moisture content must be monitored during manufacture ofproducts.

[0106] While the foregoing disclosure is directed to the preferredembodiments of the present invention, other and further embodiments ofthe invention may be devised without departing from the basic scopethereof, and the scope thereof is determined by the claims which follow.

What is claimed is:
 1. A logging tool for measuring a property ofmaterial penetrated by a borehole, comprising: (a) a nuclear radiationdetector comprising alternating components of scintillating material andnon scintillating, hydrogenous, optically transparent material, whereinsaid components are fabricated to optimize detector response; (b) meanscooperative with said detector to generate a signal indicative ofradiation impinging upon said detector; and (c) means for transformingsaid signal into a measure of said property.
 2. The logging tool ofclaim 1 wherein said components are cylindrical.
 3. The logging tool ofclaim 1 wherein said components are panels.
 4. The logging tool of claim1 wherein said components comprise: (a) a plurality of axially parallelcylinders distributed within a bounding axially parallel cylinder; and(b) said scintillating material alternates with said non scintillating,hydrogeneous, optically transparent material at each interface of saidcomponents.
 5. The logging tool of claim 1 wherein geometricconfiguration and dimensions of said components are selected to optimizetotal detector efficiency response.
 6. The logging tool of claim 1wherein geometric configuration and dimensions of said components areselected to optimize detector spectroscopy efficiency response.
 7. Thelogging tool of claim 1 wherein said scintillating material is ZnS. 8.The logging tool of claim 1 wherein said non scintillating, hydrogenous,optically transparent material is plastic.
 9. A logging tool forinvestigating a parameter of material penetrated by a borehole,comprising: (a) a detector comprising ZnS and contained within apressure housing; (b) means cooperating with said detector to generate asignal indicative of radiation impinging upon detector; and (c) meansfor relating said signal to said parameter.
 10. The logging tool ofclaim 9 wherein said pressure housing is conveyed along said borehole bymeans of a wireline.
 11. The logging tool of claim 9 wherein saidpressure housing is conveyed along said borehole by means of a drillstring.
 12. A logging system comprising: (a) a borehole instrumentcomprising (i) a fast neutron source, and (ii) a fast neutron detectoraxially spaced from said fast neutron source, wherein (b) said fastneutron detector comprises at least one interface between hydrogen richand optically transparent material and material which scintillates whenirradiated with protons, and wherein (c) the geometric configuration anddimensions said hydrogen rich and optically transparent material andsaid material which scintillates are selected for a predetermineddetector diameter thereby geometrically optimizing the detectorefficiency for fast neutrons.
 13. The logging system of claim 12 whereinsaid neutron source emits repetitive bursts of fast neutrons into earthformation, and said tool further comprises: (a) a first gamma raydetector axially spaced from said neutron source at a first spacing tomeasure (i) prompt gamma radiation during said bursts of fast neutrons,and (ii) gamma radiation induced by fast neutrons interacting withnuclei within said earth formation; (b) a second gamma ray detectoraxially spaced from said neutron source at a second spacing to measure(i) prompt gamma radiation during said bursts of fast neutrons, and (ii)gamma radiation induced by said fast neutrons interacting with nucleiwithin said earth formation; and wherein (c) said fast neutron detectormeasures fast neutrons resulting from interaction of said fast neutronswith nuclei within said formation.
 14. The logging system of claim 12wherein said material which scintillates is ZnS.
 15. The logging systemof claim 12 wherein hydrogen rich and optically transparent material isplastic.
 16. The logging system of claim 13 further comprising dataanalysis means for combining said measures of prompt gamma radiation andsaid measure of fast neutrons to obtain measures of at least oneparameter of interest of said earth formation.
 17. The logging system ofclaim 16 wherein said at least one parameter comprises gas saturation.18. The logging system of claim 16 wherein said at least one parametercomprises porosity.
 19. The logging system of claim 16 wherein said atleast one parameter comprises bulk density.
 20. The logging system ofclaim 13 wherein said second spacing is greater than said first spacing.21. A method for measuring at least one parameter of a material, themethod comprising: (a) irradiating said material with high energyneutrons; (b) measuring fast neutrons resulting from interaction of saidhigh energy neutrons with said material using a geometrically optimizedZnS/plastic fast neutron detector; (c) using said measure of fastneutrons to determine said parameter of said material.
 22. The method ofclaim 21 comprising the additional step of measuring prompt gammaradiation resulting from inelastic scatter of said high energy neutronswithin said material and combining with said measure of fast neutrons todetermine said parameter of said material.
 23. The method of claim 21wherein said material comprises the environs of a borehole penetratingearth formation.
 24. The method of claim 23 wherein said formation isirradiated with pulses of high energy neutrons produced by a neutrongenerator.
 25. The method of claim 24 farther comprising: (a) providinga first axially spaced gamma ray detector within said borehole at afirst spacing from said neutron generator and providing a second axiallyspaced gamma ray detector at a second spacing from said neutrongenerator; (b) measuring said prompt gamma radiation at said first andsaid second spacings with said gamma ray detectors; (c) axially spacingwithin said borehole said geometrically optimized ZnS/plastic fastneutron detector at a third spacing from said neutron source; and (d)measuring said fast neutrons at said third spacing with saidgeometrically optimized ZnS/plastic fast neutron detector.
 26. Themethod of claim 25 wherein said second spacing and said third spacingare greater than said first spacing.
 27. The method of claim 25 furthercomprising the steps of: (a) measuring said fast neutrons at a highenergy level and at a low energy level at said third spacing with saidgeometrically optimized ZnS/plastic fast neutron detector; and (b)combining said fast neutron high energy measurement and said fastneutron low energy measurement to minimize effects of said borehole uponsaid determination of said parameter of said earth formation.
 28. Themethod of claim 25 wherein said gamma ray measurements and said fastneutron measurements at said high energy level and said low energy levelare made during said neutron pulses.
 29. The method of claim 25 whereinsaid at least one parameter comprises gas saturation.
 30. The method ofclaim 25 wherein said at least one parameter comprises porosity.
 31. Themethod of claim 25 wherein said at least one parameter comprises bulkdensity.
 32. The method of claim 21 comprising the additional step ofenergy discriminating a response of said geometrically optimizedZnS/plastic fast neutron detector so that said detector responds only tofast neutrons impinging thereon.
 33. A method for measuring a propertyof material penetrated by a borehole, comprising: (a) placing a nuclearradiation detector within said material, wherein (i) said detectorcomprises alternating components of scintillating, material and nonscintillating, hydrogenous, optically transparent material, and (ii)said components are dimensioned to optimize detector efficiency; (b)providing means cooperative with said detector to generate a signalindicative of radiation impinging upon said detector; and (c) providingmeans for transforming said signal into a measure of said property. 34.The method of claim 33 wherein said components are cylindrical.
 35. Themethod of claim 33 wherein said components are panels.
 36. The method ofclaim 33 wherein said components comprise: (a) a plurality of axiallyparallel cylinders distributed within a bounding axially parallelcylinder thereby forming interfaces; and (b) said scintillating materialalternates with said non scintillating, hydrogeneous, opticallytransparent material at each said interfaces.
 37. The method of claim 33comprising the additional step of selecting geometric configuration anddimensions of said components to optimize total detector efficiency fora specified detector diameter.
 38. The method of claim 33 comprising theadditional step of selecting geometric configuration and dimensions ofsaid components to optimize spectroscopy efficiency of the detector fora specified detector diameter.
 39. The method of claim 33 wherein saidscintillating material is ZnS.
 40. The method of claim 33 wherein saidnon scintillating, hydrogenous, optically transparent material isplastic.
 41. A method for investigating a parameter of materialpenetrated by a borehole, comprising: (a) mounting a detector comprisingZnS within a pressure housing; and (b) generating a signal from anoutput of said detector which is indicative of radiation impinging upondetector and thereby indicative of said parameter.
 42. The method ofclaim 41 comprising the additional step of conveying said pressurehousing along said borehole by means of a wireline.
 43. The method ofclaim 41 comprising the additional step of conveying said pressurehousing along said borehole by means of a drill string.